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What is the slowest fixed-wing airplane?
It's easy to find information about the fastest airplanes, in different categories (e.g. X-15, SR-71, the Concorde etc), but what is the slowest one? Which powered, manned airplane is capable of sustained level flight at lowest velocity?

A hovering helicopter is an aircraft! An airship is an aircraft. Are you asking only about fixed-wing powered aircraft?

All VTOL will tie for the win.

I wonder if any of these would take the prize?

Nobody has mentioned the military jets such as the Harrier which can hover (or even fly backwards) using vectored thrust. I guess this is "cheating" in the context of this question, or possibly not sufficiently sustained before it consumes a full load of fuel.

@nigel222: It needs to be an airplane (sustained by lift from wings, not by thrust) else a helicopter would also be relevant, and maybe Elon Musk rockets.

Note that an autogyro is kept aloft by lift from its wings, which (unlike the rotor blades of a helicopter) are not powered; a separate propeller provides forward thrust to keep the aircraft moving forward,which causes air to flow past the wings. And an autogyro can fly very slow. But since the wings autorotate, the autogyro is not considered a fixed wing aircraft; it's a footnote to this question rather than an answer.

I propose that to solve this puzzle we have a race and see who comes in last, with the owner of that plane receiving a(n) hefty prize... and each person will fly a plane that isn't their own.

They aren't going to be winners in this competition, but two curiosities that would make a fine addition here would be the "Flying Pancakes" (Vought V-173/XF-5U), which had extremely unusual wing geometry and very low stall speeds by design. The XF-5U, which had been built, taxi-tested but not really flown, would have had stall speeds ~20 kts.

Hmm... This question says "at lowest velocity." Did you perhaps mean "lowest airspeed?" It's quite possible to sustain flight with negative forward velocity (relative to the ground) in a strong headwind in a fixed-wing aircraft.

I've had people tell me they landed a C42 backwards, so less than 0kts forward speed :)

The Gossamer Albatross is a human-powered plane with a top speed of 29 km/h (18mph).

It was used to cross the English Channel and seems to meet the criteria of the question.

Well... http://newatlas.com/mow-cycle-human-powered-riding-mower/14491/

This aircraft is as much powered as... the two bicycles below it ;-)

It's very Kiki's delivery service  :)

Is there any info about the stall or minimum speed?  https://aviation.stackexchange.com/questions/39327/whats-the-slowest-airplane#comment103345_39373 suggests it can fly at 7mph, but is there anything more definitive?

The 7 mph from that answer was for the Gossamer Condor, the predecessor of the Albatross.

The Antonov AN-2 has no stall speed quoted in the operating manual and can fly under full control at about 30 mph. Thus if the headwind is sufficiently large the aircraft will move backwards with respect to the ground.

DHC-2 Beaver is similar. Normal landing speed is about 45mph, meaning it can be operated significantly slower than that.

I've flown one (as a passenger) on a sightseeing flight. Nice machine, albeit not the most comfortable one.

@el.pescado me too - just about.  It was the first time I had ever been in an aircraft without large tubofans, and if my wife was not with me to relate the tale to my friends after,,  I would have bailed out of the thing before the takeoff run:)   The rising whine of the inertia-wheel starter brought on a sort of panic attack...

@MartinJames it had been my first filght in my life, and it had been years before commercial flights become affordable for average Eastern Europe citizen. That means, until relative recently, my impression of flight was based on that An-2 flight;)

Does that mean a 30mph wind will lift the airplane off the ground?

@Chloe, that is why they strap-down light aircraft if heavy winds are expected:
http://www.pilotweb.aero/polopoly_fs/1.4881486!/image/image.jpg_gen/derivatives/landscape_630/image.jpg

I saw one of these at Cable airport in California in the 90ies. It was huge, and yellow. The best part is that you can fly it with nothing more than a private pilot license and a tailwheel endorsement.

Here you can see it happen; not sure if it actually goes backwards, but comes very close.

if the headwind is sufficiently large the aircraft will move backwards with respect to the ground technically, this is true of all aircraft.

The Harrier, Yak-38, Yak-141, XV-15, and V-22 are all fixed wing aircraft. All can hover in mid air, controlled.
So they are in controlled flight at 0 velocity.
At least the Harrier can even be in controlled flight flying backwards, so with negative velocity. The others may as well, I don't know.

I should have expected this;) How about "ordinary", non-VTOL aircraft?

Another trick answer would be - small model aircraft.  In fact, do those things fly slowly??  Or does it not scale like that?

@Fattie the requirement for the airplane to be manned may exclude all but the largest model aircraft, not even considering whether manning them would be safe, legal, feasible, ...

Oh sorry - I missed that requirement.

You could also use the trick of requiring a very small "man" (see Jessica Dubroff).

From what I understand, the fuel consumption per minute on those jets is much higher when they are hovering than when they are cruising.  If one were to rank planes by the airspeed that maximizes time aloft per unit of energy, I wonder which plane would rank lowest?

@supercat probably something like a 787 or A350

Nah, the velocity is never negative unless it is in relation to a fixed point, such as the destination....

Don't forget the F-35B bro!

@JustACoder that one didn't exist at the time this answer was written :)

Well it actually did exist 4 years before this question was written:
https://www.youtube.com/watch?v=zW28Mb1YvwY

now, if you're looking at modern, more commonly used transportation, powered paragliding would probably take the cake.

CC BY 3.0, https://en.wikipedia.org/w/index.php?curid=13110495

Powered paragliders usually fly between 15 and 50 mph (25 and 72 km/h) at altitudes from 'foot-dragging on the water' up to 24,000+ ft (5400 m)

https://en.wikipedia.org/wiki/Powered_paragliding
The beginners equipment are often the slowest, and with proper skill, and the right weather, can have a stall speed of zero. Whilst being lifted by a thermal, the only thing that will push the glider forward is the natural tilt. naturally, on a still day, the slowest of equipment will stall at below 10mph.
Disclaimer: this is not an airplane, but it is a fixed wing vehicle, made to primarily move forward to generate lift, akin to most airplanes.

Further comments regarding the appropriateness of this answer to the question can be found in chat.

"The beginners equipment are often the slowest, and with proper skill, and the right weather, can have a stall speed of zero" is patently wrong. It can have ground speed of zero, but that's true of any wing with sufficiently strong headwinds. But it will never have an air speed of zero (or even infinitesimal) and keep generating lift, which is the only thing referenced when discussing stall speed.

@mathrick actually, doing special maneuvers can allow you to paraglide backwards, even on days with no wind! depending on what OP means by slowest, this may be allowed!

When you're flying backwards, that's because the wing is either stalled, or sinking. I dunno how paragliders accomplish that, but in parachutes (specifically, BASE canopies), sustained straight-down / backwards flight is accomplished by having bottom-skin vents which allow the wings to pressurise and effectively change the relative wind direction. In non-vented accuracy canopies, straight down (but not backwards) flight is possible in deep brakes, but even then the airspeed is not zero, it's positive and a high angle of attack. Even if it's sinking, the parachute is still not stationary.

@mathrick Well, here's the thing. From an airspeed prospective, the only thing that paragliders do is sink. I think it may be going over your head how this works.

Sure, and they do it at a non-zero speed, any time the wing is flying.

@mathrick I think that if we're using the same definition of flying (flight) as the answer with the VTOLs, I don't think that we even have to be moving to be flying. Maybe they're using flight incorrectly. I don't think so.

If you are including historical aircraft, the Wright Flyer averaged 10 fps (approx 6.8 mph or 11 kph) over it's first 120 foot flight in 1903.
Reference: https://airandspace.si.edu/exhibitions/wright-brothers/online/fly/1903/triumph.cfm

Wind did a lot to make this slow speed possible.

What is "kph" for a Unit? Or did you mean km/h?

@12431234123412341234123 Yes, kph and km/h are the same thing: kilometers per hour.

Did anyone else see "10 fps" and think "10 frames per second"?

Tony - no, nobody thought that  :)

Peter: I saw no info about wind direction, didn't know if it was a headwind or a crosswind. Since the flight was northbound, I would have assumed crosswind. Per the linked site, subsequent flights had a higher average speed. Tony: funny. :)  Highlights the importance of context.

@tony, yeah, frame rates were crappy back then

When I saw "10 fps", I was thinking "ferrets pedaling slowly".

@GalacticCowboy The thing with kph is that it's not actually used all that much in countries that use kilometers. In fact, it mostly seems to be used by north Americans who try to to convert mph into metric.

@AndrejaKo It's also widely used in the UK; it's a legitimate abbreviation in English.

Gossamer Condor, the first human powered airplane capable of basic maneuvered flight.  When it finally won the Kremer prize for a 1 mile figure 8 course it did that course in 7 minutes and 22 seconds.
The later Gossamer Albatross that crossed the English Channel had to be faster.
This assumes that the question criteria was AIRSPEED (not groundspeed) for a fixed-wing, man-carrying aircraft that could take off and land under its own power and maneuver both into and out of the wind (figure 8 maneuver).

Wow. 7mph! That's slower than most paragliders

Building it just for very slow speed does not look practical (if you really need this, use helicopter). Some old planes may be slower, but they do not use the newest technologies and may not be built for slow flight anyway. Some patrol, agricultural planes may benefit from slow flying, but we need something more extreme.
I expect such aircraft to be some specific machine that has a huge wing and low weight for other reasons, and should be recently built to benefit from latest technologies.
Solar impulse looks like a good candidate. It has take-off speed of 35 km/h (22 mph) only. Looks like its minimal speed is about 20.67 mph only. Its large wing holds the solar batteries. It has a wingspan larger than Boeing 747 and the weight close to Cessna 172!

(picture from Wikimedia Commons).

Agricultural planes can fly very slow in general

Even if not designed to fly slow, some airplane simply has to be the slowest one.

Some aircraft were designed with good low speed performance in mind, either because helicopters didn't exist at the time or because they aren't practical for the job the aircraft was designed to perform. The Tiger Moth comes to mind, and indeed some models of crop duster aircraft. The DHC-2 Beaver was designed with very slow stall speed to allow it to land in remote areas with hardly any flat space to use as a runway. Examples are legion.

@el.pescado The math pedant in me wants to add "assuming there are only finitely many airplanes". ;-)

@Charles actually, there might be infinitely many airplanes, it's only neccessary that every totally ordered subset has lower bound ;-)

@el.pescado If the speed of airplane number x (where x is any natural number) is given by the formula 30 + (1/x), which airplane has the lowest speed?

@ChrisW how many mathematicians does it take to fly an aeroplane? ;-)

@ChrisW BTW that set of airplaines doesn't satisfy Zorn lemma (linked by me) preconditions.

The Gossamer Albatross had a top speed of 18mph, and an average speed of 7.8 mph.

The Ruppert Archaeoptrix Electro (Wikipedia, official website) apparently has a stall speed of 30 km/h (19 mph / 16 kn), and I think that makes it a candidate for the current "slowest" fixed wing aircraft.
It is a relatively new glider that can be foot launched, but there are also wheeled and motorless configurations, and it can also be launched by towing.

For the motorized version:

Which powered, manned airplane is capable of sustained level flight at lowest velocity? While your answer does not cover "powered" I love the picture.

That plane somehow reminds me of the Flintstones' cars...

@KorvinStarmast It has in fact a motor, which is not used in this picture. I recommend taking a look at the manufactureres website =)

The preview image of the video shows one. It doesn't look that much different, I just has a folding prop sticking out of the back, also e.g. here: https://upload.wikimedia.org/wikipedia/en/b/bb/Archaeopteryx_electric_motor.jpg

The first picture looks like someone's very elaborate Halloween costume.

You should check out planes with custer wings.
https://en.wikipedia.org/wiki/Custer_CCW-5
It was claimed that the aircraft could fly under control at 11 mph (18 km/h) and that it could take off with a 1,500 lb (680 kg) load at 70% power in 90 ft (28 m).

Welcome. Interesting addition that should be a comment, not an answer.

Seems pretty answery to me, @mins. Addresses all the items in the OP: Powered. Fixed wing. Sustained flight at very slow air(?) speed. Has a link for more info, but provides the necessary facts in case Wikipedia ever dies...;)

@FreeMan: The question is about existing airplanes.

@mins: The question isn't very explicit about that.  It does use the present tense, but seems to leave the door open for interesting answers about past planes as long as they really flew, and weren't just designs on paper.

Thank you for your feedback. Even if it's not used anymore in civil or military aviation, the concept of channel wings is still used for modelism or prototypes (Handiwork 181, in 90s).

Feels like this plane has (at least) two relevant airspeeds. The 11 mph is likely not the airspeed inside the channels?

Slepcev Storch
(A 3/4-size replica of the Fieseler Fi 156 Storch, a German WW2 reconnaissance aircraft well known for its slow-speed performance)
36 km/h or 40 km/h (19kn/22kn)

Picture Source

Performance according to Wikipedia:

Maximum speed: 150 km/h (93 mph; 81 kn)
Cruise speed: 133 km/h (83 mph; 72 kn)
Stall speed: 40 km/h (25 mph; 22 kn)
Rate of climb: 6 m/s (1,200 ft/min)

Performance according to ulm.it:

Maximum speed: 150 km/h
VNE: 182 km/h
Stall speed: (flaps out) 36 Km/h
Climb rate: 9,1 m/s (1800 ft/min)

If the question involves non experimental fixed wing aircraft that were actually in general use, the Storch would have been the one I would think of.

I was at the Biggen Hill airshow many years ago where not one but two different aircraft flew backwards. One was the AN-2 which was able to nose up into the headwind so far that it actually flew backwards for a short distance down the runway. The other was a Russian super jet, (possibly a Sukhoi 27 or Sukhoi 35?) that did a vertical climb on afterburners that shock the ground then slowed to a stop before falling on its tail while still vertical. It flew tail first towards the ground before lowering the nose.

That manoeuver is called Pugachev's Cobra.

@el.pescado Thank you. That was the stunt he flew that left me stunned. As you can imagine, there was no way to hear what the commentator had to say.

Here is a video showing that the Gossamer Albatross flew at lowest speed, 7mph (11.2kmph). Very slow. But the save speed will be 10mph (16.1kmph), as mentioned in that video. It lasted 2 hours to cross England strait from England to France.

Alaskan Bush Planes are modified to fly very slowly for short landings:

Features include:
"Short runway requirements, typically gained through high aspect ratio wings and high-lift devices such as flaps, slots and slats to improve low speed flight characteristics, allowing shorter ground rolls on landing or takeoff."
https://en.wikipedia.org/wiki/Bush_plane
This might be helpful too:
https://en.wikipedia.org/wiki/List_of_STOL_aircraft

According to Review of Human Powered Flight to 1990 (PDF) by Chris Roper, the MIT Chrysalis human-powered aircraft "was in fact the slowest aircraft ever to fly," at 8mph.  The document covers other human-powered aircraft mentioned in other answers, including the Gossamer Condor and Albatross, so that suggests that the MIT Chrysalis was, in fact, the slowest fixed-wing manned aircraft to fly successfully.

It is the shock ultra by savage.  Stalls at 18 mph.    2016 savage produced it and it by far out flies all other planes by a mile in the low and slow cathegory

Fairly sure a stall speed of 18mph doesn't beat the An-2's lack of a stall speed.

@Jules, this is a myth. First, An-2 does stall; it just remains somewhat controllable and very stable. This mode is called 'parachuting'; An-2 can 'parachute', fully stalled, to the ground and survive. Second, An-2 can sustain flight at 40 km/h (~21 kt) the lowest (more practically 50-70 km/h, depending on the age and configuration). At anything slower it will descend.


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Where can I find climb and cruise performance charts for a specific aircraft?
I'm looking for climb and cruise charts for a cirrus sr20 G3, but I can't find them in the POH?  where are these charts typically located?

Does the SR20 come with a separate Air Crew Manual or perhaps a Performance Manual? I would normally expect to find such information in Section 5 of the POH, otherwise. [full disclosure: I've never checked out in a Cirrus]

You generally only find that kind of stuff in transport category airplane pubs where there is a performance engineering organization to create the data.

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How to calculate the performance parameters of an hexacopter (UAV)?
I need to calculate the performance parameters like cruise speed, endurance, range of my hexacopter but how can I do that!
Also, is there a way to obtain motor-propeller performance parameters without using a load cell?

Welcome to Aviation.SE! I would suggest you start by reading the [help] to get an idea for how this site works. You should generally try to ask one specific question at a time and provide as much detail as possible, including what you already know and what you have tried. As it currently stands, your question is likely to be closed. You could prevent this by using the edit button to improve your question.

The way it's done in helicopters is by using power vs speed curves, comparing required power and available power. You would need to compute:

Rotor induced power
Rotor profile power
Fuselage aerodynamic drag power
Tail rotor & transmission power (probably  not applicable in your case)
Power loss from friction, electrical heat etc.

Total power to remain airborne is a function of gross weight, air density, rotor state, aircraft state and velocity vectors.
There is a way to measure delivered power of an electric motor, I assume that is what your copter uses: measure delivered amperage for the applied torque, measure voltage over the motor for rotational velocity, and multiply the two for applied electrical power.

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Where can I listen to the actual cockpit voice recorder audio from US Airways Flight 1549?
I have searched for the audio recorded on the CVR from the US Airways Flight 1549, but all clips and audios I found seems to be audio recorded from the flight control and not from microphones inside the cockpit.
For example:

In this clip, we only hear the pilot sparsely, but the air traffic control and the airport tower controls quite often. Although, we can also see a transcript of what is said in the cockpit, so there obviously exists some recordings from the cockpit.
Where can I listen to the audio recorded from the cockpit and not from the air traffic control?

You can't, because it is in fact forbidden to release CVR audio under US law. From the NTSB:

The CVR recordings are treated differently than the other factual information obtained in an accident investigation. Due to the highly sensitive nature of the verbal communications inside the cockpit, Congress has required that the Safety Board not release any part of a CVR audio recording.

Other countries may have different rules.
This rule came about after the crash of Delta Air Lines flight 1141, (commented by @MichaelK)

[...] where the crew — in extremely cruel irony — were bantering on the flight deck about what to say that would get captured on the CVR in case of a crash. That irony was then squared in that the bantering was a directly contributing cause of the crash, in that they forgot to deploy flaps and slats

You can't listen to it, but you can read the transcripts. This rule is mostly to avoid the stress/grief caused to families as they listen to the last minutes of a loved ones life.

Trivia about that: this decision came about after the fatal accident of Delta Air Lines 1141 in 1988, where the crew — in extremely cruel irony — were bantering on the flight deck about what to say that would get captured on the CVR in case of a crash. That irony was then squared in that the bantering was a directly contributing cause of the crash, in that they forgot to deploy flaps and slats.

Good Lord, what a story, @MichaelK

@Fattie Want to know the final rub of salt in the wound? It was the CVR recording that revealed that the flap and slat points on the checklists had not been stated by the crew, and that there were no sounds from the handles being moved... thus presenting solid evidence that the flaps and slats were not extended. This recording was then played on the news, the crew's error being laid bare for all hear. As a direct result of this, legislation came into place that the NTSB may not release recordings from CVRs.

Good Lord man!  Actually, an interesting point then, others have suggested the law was made to "spare feelings" (a worthy goal), however more cynically the information you relate suggests it was really made to spare embarassment to the faulted!

It's ironic that this should happen in a country with freedom of speech as the first amendment to its constitution ;)

The 1st amdmt protects a person from being told what to say by the State. It doesn't mandate that investigators have to broadcast the final words of people to the public.

@alephzero Not releasing something isn't a freedom of speech violation. Banning someone from publishing it if they had it might be, but simply not giving it to them in the first place isn't.

"highly sensitive nature" - "[extremely embarrassing and costing us money, so we all got together and lobbied Congress to make it illegal]"

@alephzero What's actually relevant is the 4th Amendment: the right against unreasonable searches by the government. The CVR is a government mandated recording device whose contents could be used against somebody in a legal proceeding. An additional law (49 USC 1154) only allows transcript admissibility only if necessary for a fair trial.

@alephzero What user71659 said: a CVR is a government mandated eavesdropping device, in a workplace. Just extrapolate this in absurdum and imagine if every office and workplace had this; that the government always had recordings of what everyone said in their workplaces. Normally we would never allow such a thing. The CVR is an exception where we find that the use of a CVR outweighs the privacy rights of those that have to endure its presence. As such it is only natural that recordings from CVRs cannot be released willy-nilly.

@MichaelK Pilots generally press a button to wipe the CVR on landing, so the recording is not going to be accessible unless there is a crash.

@forest That does not matter. It is still a government mandated eavesdropping device, and it is in your workplace. Constitutionally this is normally not possible, so it is their existence that is the exception, not the prohibition on releasing the audio.

@MichaelK Also honestly it seems like kind of a cruel irony, considering the physical facts of the crash, that the NTSB report's front page depicts an airliner with its right wing dipped.

@LightnessRacesinOrbit A T-tailed airliner none the less... with a flaps & main gear configuration suspiciously like that of a B-727.

@MichaelK Good grief what a flight!

Yes, and Air Canada 781's aircraft was allowed to continue flying, so by the time it escalated to management that the incident was serious, the plane had flown another umpteen hours and the data was overwritten.  So we'll never know what happened in that cockpit.

@Mazura Absolutely. How convenient. https://web.archive.org/web/20150711081847/http://archive.news-leader.com/article/20051009/COLUMNISTS11/510090341/-State-secrets-protected-by-lie

@Chloe - I didn't know Blue Thunder was a true story. (the plot of the movie revolving around retrieving a helicopter's black box and the cockpit recording on it of the bad guys talking about criminal intent, whom they were clandestinely observing ;)

You can't listen to it due to the regulation mentioned in Sanchises' answer, but you can find the transcripts. There is one here, for example.

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Is every emergency landing of a commercial flight documented?
A friend told me she was on a Royal Jordanian commercial flight in 2006 that had an emergency and was forced to land in a field in Greece.
She said the final destination was supposed to be Italy, with no scheduled stop in Greece. Nobody was hurt, and passengers were transported by bus to the Athens airport, which was not far. Perhaps the pilots had been attempting to reach Athens for an emergency landing.
Neither her nor I know much about aviation, but she described the plane as a small commercial jet holding 50-70 or so passengers.
I would like to know whether this kind of incident would create some kind of publicly-accessible record that I could look up. A commercial flight landing in a field seems significant enough to be recorded. But maybe as long as there were no injuries it isn't?

In the US this would be a recordable (and searchable on the NTSB site) incident. I'm not sure about Jordan/Greece since they treat those types of investigations differently. At the very least there should be some news stories about it from that time.

Language nitpick: "Ditching" specifically refers to a landing on water. If the aircraft landed in a field, it didn't ditch.

@HenningMakholm, it's more than language nitpick—if the story can't even decide whether the aircraft landed on water or solid ground, how good is the memory for the other things?

Technically speaking, every landing is documented, even the normal one. Every hour of flight or engine running is logged; the pilots have a logbook where they record the hours and route; the airline must have a record. It's all traceable, but not necessarily publicly available.

Generally there should be some kind of record of this incident but even more broadly speaking not all aviation accidents are necessarily documented or reported on a global scale.
It depends on what country you are in and who the reporting agency is. Here in the USA the NTSB is responsible for investigations of aircraft incidents. Investigations and reports are generally the responsibility of the country in which they happen but not all countries have investigation agencies nor the means to carry out complex investigations. Historically the manufacturers as well as other nations have offered assistance when need be as the outcome is as important to them as it is to the investigating agencies.
As for Greece their reporting agency can be found here and the list of 2006 reports here although they seem to publish the list by year that the report came out (not the year of the accident) so the 2007 list may also be of interest. Most of the reports are published in greek which I cant read and there does not seem to be one relating to Royal Jordanian.
Not that it is a complete list by any means but wiki does not list any similar Greek aviation incidents on their compiled list.
For the sake of completeness, what appears to be a grass field to some may actually be a grass runway (more common than you would think) and its possible what your friend described was a landing at a grass runway (field, or strip) done for a precautionary reason. A diversion like this may not cause an investigation to ensue.

By "a field", it's possible that they were referring to an airfield, not necessarily an actual grass field.

Investigation often take several years, but I went through the list all the way to 2013 and didn't see anything Jordanian.

As per ICAO guidelines, all accidents¹ and incidents² should be reported. Accidents should be investigated, whether incidents are is at the discretion of the agency collecting the reports.
Now whether, and how carefully, the investigation is done, depends on the country, but Greece is a functioning state and EU member, so chances of missing an accident there is pretty low.
Besides the web of the appropriate agency (already linked by Dave), there are two services that aggregate the reports, at least in the more serious categories:

The Aviation Safety Network, https://aviation-safety.net/
The Aviation Herald, https://avherald.com/

They may not have everything, but the ASN should be fairly complete as far as accidents with any significant damage go, even including private and military flights. The AvHerald only concerns with commercial flights, but it also has a selection of incidents and the kind of “lesser” accidents like turbulence injuring some passengers or crew members.
In any case, something as severe as forced landing, be it ditching—which means landing on water—or off-field landing on ground, in a functioning state like Greece, would certainly make it to both, and a bunch of general news outlets too.
However, there are no commercial flight accidents for Greece in 2006 in ASN (the one listed accident is a fire-fighting aircraft), no accidents for Royal Jordanian Airlines in 2006 in ASN and search on AvHerald does not return anything either.
So it seems either your friend does not remember the place, airline or year well, or it was not such a big deal—perhaps the aircraft just diverted to some small airport due to a technical issue, which would be just an incident and wouldn't have to make these lists.

¹ An accident is any occurrence where somebody is killed or injured or there is substantial damage to the aircraft (excluding engines and landing gear) or other property.
² An incident is any occurrence where safety of operation might have been compromised (but didn't turn into an accident).


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Can the airport name be omitted when contacting ATC if the frequency is not used by any other nearby airports?
I was thinking of contacting Clearance/Ground/Tower/approach and omitting the airport name to shorten the transmission. If the freqs are not used by any other nearby airports, I figure there would be no confusion which airport I'm talking to.  Is there any other concerns that make this not a good idea?

Even if in some jurisditcion it was legal, this is a horrible idea. Beside radio set to wrong freq, also propagation can do magic tricks. At my local flying club we found out after years we where not alone on that frequency in a day of great propagation, when we started to hear from guy 600 miles south. (!!!)

The rules are the rules because people can't be trusted to think correctly.  As a good general rule, any time you think you have an idea that is better than the rules, you're probably wrong.  Unless you enjoy the notion of regular post-flight phonecalls it's probably best just to do things the right way.

When every transmission to the tower begins with "Executive Tower...." that is the keyword that makes the controller's ears perk up and start taking notes.
If you don't say the magic words, your controller may be a step or two behind as you rattle off your request.

Also, you think you're on the right frequency, and you think you know who you're talking to, but what if you mis-tuned the frequency knob by a digit or two, or forgot to hit the COM1/COM2 button?  You could be talking to the wrong receiver and not know it.  That is how airplanes sometimes land at the wrong airport.
Every initial transmission should start with the intended receiver; that is part of the rules and just common sense.  Don't try to shorten transmissions and end up causing more work for everyone.

You didn't add any jurisdiction, and I don't know if this applies to the US (though I suspect it does at least to some extent), but in Europe, it's also pretty common for the controller to shorten call signs.
So your initial call on a small European airport might be as Springfield Tower, AB-XYZ, only to have the controller come back to you with A-YZ, Springfield. In that case, you're expected to keep using the abbreviated call signs when communicating with that station or facility, so your response will be addressed as Springfield, A-YZ for the duration of your contact with Springfield Tower. When you leave their frequency or otherwise stop communicating with them, then call them up again later, you start over with Springfield Tower, AB-XYZ to avoid ambiguities.
The same principle applies also with controllers talking to many more aircraft; at least in the case of Sweden, all the way up to Sweden Control.
The controller (hopefully) knows which stations (such as aircraft) they are communicating with at the time, and are in a much better position than you are to determine how much a given call sign can be shortened without risk of conflict. Rather than making guesses about this yourself, let them take the lead. You're much less likely to make a mistake that way.
Besides, it has to be a very long airport name if it takes more than a second or two to rattle off, so it's not like you're saving all that much for the risks you get by omitting it.
Given that the overall aim of radio communication in aviation is to increase flight safety, a good rule of thumb is to simply ask yourself if doing something, or doing it in some particular way, is conducive to improving flight safety (either in the air or on the ground) compared to not doing it at all, or doing it in some other way. If it doesn't improve flight safety, then as a rule of thumb you should probably refrain.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Is it possible to obtain the lift and drag coefficients of a 3D body in ANSYS Fluent?
Is it possible to obtain the lift and drag coefficients of a three dimensional body in ANSYS Fluent? Or, would it be acceptable to take the coefficients of several cross-sections along the wings and body and perform a weighted average calculation to find the lift and drag values for the entire craft?

For sure you will get results but how accurate do you need to be? Fluent (and any CFD code) has severe restrictions and assumptions. Also, all CFD codes require a specialist (if not expert) in order to give reasonable / acceptable results, for non-basic problems.

Yes this is possible, and it is the way lift and drag of aircraft are estimated in the pre-design phase. There is detailed methodology described in Synthesis of Subsonic Airplane Design by E. Torenbeek, Appendix E. for lift and pitching moment, Appendix F. for drag.
For lift and pitching moment, first the wing characteristics are estimated based on known research and on corrections for fuselage length, wing/body interference length etc.
For drag, first the $C_{D_0}$ of all components in the table underneath are determined, and then the lift of the wing, body, tailplane etc as function of angle of attack are added.

Yes, its possible to get directly the drag and lift force for entire 3D body in Fluent, and its easy. See this Fluent user guide page.
You can use the other method you mentioned too, if you know what exactly you are doing while taking weighted average.


text:
Can a helicopter like the R44 be lifted up by his rotor head?
I am curious if a rotorcraft could be suspended from rotorhead for an extended period of time.
I am curious about R22 and R44.  I believe I have seen them in the shop lifted up from the 2 holes in the rotor head.
I am also curious if all helicopters have mount/lift points in the rotor head like an MD500C. Would you be able to do the same?
Or are you better lifting from the skids for a longer term lifting?

I‘m pretty sure all helicopters are suspended from their rotors when flying. Seems to work for most of them.

@JustSid Bearings can be rated for lower static loads than dynamic loads - when flying, your rotor is definitely not static.

And if you're looking for the lift points, best to consult the specific aircraft type maintenance manual to find out precisely what to do and how to do it.

I was just in the R44 and can confirm, it is lifted via the rotors

Yes, but you better know what the heck you are doing. :) Whether an individual helicopter can be lifted this way is a different question. You'd have to look at the maintenance manual for the individual aircraft for the answer.

Note the crossbar: The side loads of a crossbar-less hoist would definitely crush the rotor mounts.

@PeterKämpf Absolutely. Hence my "...you better know what the heck you are doing." comment! LOL! :)

That comment applies to just about everywhere. And is often enough neglected …

...and usually preceded by "Watch this!" in any number of languages. :)


text:
Are there rules about using the "Fasten seatbelt" sign outside of take-off/landing/turbulence?
Related to this question regarding flight attendants continuing to stand during turbulence.
My understanding is that it is the law for passengers to be seated with their belts fastened during taxi/take-off/landing along with the flight attendants.
At other times during the flight, the fasten seatbelt sign is also illuminated at times of turbulence/expected turbulence (as per the question linked above, and usually accompanied by a warning from the captain that they're expecting turbulence) - I assume this is subject to the same rules that insist passengers are actually seated during these periods.
On several recent flights on budget airlines (Ryanair/EasyJet), I've begun to notice that the crew are using the fasten seatbelt sign in order to ensure passengers are seated whilst they're serving food/drinks etc. This was confirmed when I was sat at the front of the plane and over heard the cabin crew discussing this.
Clearly, there is a good rationale for keeping passengers seated whilst hot drinks are being served as the aisles on budget planes are too narrow to easily squeeze past, but on Ryanair/EasyJet flights there are also significant times when cabin crew are simply trying to flog cheap perfume and scratch cards.
During these recent flights where the fasten seatbelt sign was illuminated unnecessarily for long periods of time, passengers begun to simply ignore the signs in order to make use of the toilets (especially people with young children etc).
Are there rules that govern when pilots/crew may illuminate the fasten seatbelt signs, and are the passengers breaking any laws by ignoring them?

You dont have a jurisdiction tagged (although it sounds like this is an EASA matter) your answer may vary depending on where you are asking for. I can answer for the US/FAA.
Here in the states you are technically in breach of an FAA FAR by not remaining seated. As per FAA FAR 121.317

Sec. 121.317
Passenger information requirements, smoking prohibitions, and additional seat belt requirements.
....
(f) Each passenger required by Sec. 121.311(b) to occupy a seat or
  berth shall fasten his or her safety belt about him or her and keep it
  fastened while the "Fasten Seat Belt" sign is lighted.
.....
(k) Each passenger shall comply with instructions given him or her by a crewmember regarding compliance with paragraphs (f), (g), (h), and (l) of this section.

I cant find a regulation on using it in times its not otherwise required by FAA FAR 121.311 but this discussion seems to elude to the fact its at the "Captains Discretion".

So you can send your 2-year-old "lap child" to the bathroom on his own since she's not technically required to wear a seat belt?  :D


text:
What happens during a carrier take off?
Watching videos of jets taking off from a carrier, it appears the pilot is holding a hand grip as the jet is catapulted along the deck; then there is 'jerk' type motion as the jet leaves the deck with the pilot then taking control.  Could you please describe what happens during the take off.  It looks like a really rough ride for the pilot.

Can you provide a link to one of these videos so we can see exactly what you're describing?

A carrier aircraft launcher is called a "catapult", for good reason! It's job is to get an aircraft up to flying speed very quickly, which of course results in a very high acceleration for the plane and pilot. Sometimes as high as 4G.
On modern American aircraft carriers, the catapult is about 300 feet (90 metres) long, and can get a large plane up to flying speed, 140 knots, in about 2 seconds. By comparison, taking off on a conventional runway a plane might need 2 kilometres to get up to this speed.
As you can imagine this puts a lot of stress on the planes, carrier aircraft are built much more rugged than an aircraft that would only fly off normal runways.
Each catapult-launched aircraft has different procedures for launch. In the case of the F/A-18, the pilot keeps his hands off the control column during the actual launch, grabbing it as soon as the plane leaves the edge of the deck. This is not the case for other carrier aircraft, such as the C-2 Greyhound, nor is it the case when taking off the same aircraft from a runway.
I found this in the F/A-18E/F flight manual (PDF here):

Immediately after the end of the catapult stroke, the aircraft will rotate to capture the
12° reference AOA (hands-off). To avoid PIO with the FCS, do not restrain the stick during catapult
launch or make stick inputs immediately after catapult launch. The pilot should attempt to remain
out of the loop but should closely monitor the catapult sequence.

AOA = Angle of Attack, PIO = Pilot-induced oscillations, FCS = Flight Control System.
Which roughly translates as "The F-18 will fly itself and find the optimal climb angle without pilot input during the launch."  This is not the case though for other aircraft.

What you see here as well as here is the pilot grabbing the handle during takeoff. According to this it is to keep the pilots hands off the controls due to the fact that the natural response to the maneuver would cause the pilot to dive the plane. There is also some discussion on it here which offers a counter point:

No that's not why you hold the towel rack. Yes that inner ear
  sensation can happen, but that's not why. What about the F-14, or A-7,
  or literally every other airplane that got shot off the front of an
  aircraft carrier that required the pilot's hand to be on the stick to
  fly it away?
The reason you don't touch the stick in an F-18 is because the F-18 is
  very sensitive to pitch inputs at slow speed in the takeoff
  configuration. If you ham-fisted it, you could pull too much angle of
  attack too soon and stall.

The reason isn't pitch sensitivity, it is because the autopilot is programmed to rotate to the proper attitude and it will cause PIO if you are also trying to fly.

P.S.  The discussion on the "according to this" link is incorrect, but I don't have a reddit account to counter.  The "some discussion on it here" link takes you to advertisements.

All pilots salute to signify they are ready for takeoff.  They aren't even supposed to put their arms above the canopy rail to avoid accidental shots.
There are hold bars on the throttles of some planes to avoid a situation where the throttles go to idle during the shot.
With the introduction of fly by wire the 18 can have a computer controlled launch and the pilot holds the handle during the shot.
That jerk you see . . .
There is a tremendous amount of pressure on the tires during the launch from down force produced by the elevators.
With the 18 you can see that the elevators increase deflection gradually during the shot whereas most other launches have the elevators at full deflection throughout the cat shot. As the plane leaves the deck the back of the plane is forced down.
If you watch video where the airspeed is visible you will see that it momentarily drops when the plane first leaves the deck.

Adding a link to a video in your answer would be good. You can edit the question and use the Link symbol.

“Most other launches” do not have the elevator at full deflection. There is a trim setting for launch, and at the end of the stroke, (non- F18) the pilot will rotate just like a normal takeoff.

In the early days of catapault launches they discovered that the pilot could not withstand the inertia of his hand slamming backwards at the time of launch. If that hand is holding the stick, then you end up with a large pull back, and thus a large pitch up, which is deemed to be non-optimal at that stage of the flight.
So some manufacturers added something other than the stick for the pilot to hold during the cat shot. (And the USN procedures have the pilot salute the cat launch controller to show his hand's out of the way.)

Based on the other answers, the F18 will automatically find the proper AOA immediately after launch, but other aircraft won't. Are you saying that F-18 pilots are the only ones who salute before the shot? I have a hard time believing that...

"...and thus a large pitch up, which is deemed to be non-optimal at that stage of the flight."  I would agree that stalling one's plane during takeoff is generally somewhat ungood...

The salute is the universal signal for all aircraft types that you are ready to launch.  At the end of the stroke the acceleration stops, so you aren't going to over rotate from inertia.


text:
How to calculate the static pressure in a pitot tube?
In a pitot-static tube, we have two different pressure, the static pressure $p_s$ and the total pressure $p_0 = p_s + \frac{1}{2}\rho v^2$, which comes from the Beroulli equation for incompressible flows.
Now, lets say we are given an inflow of air from the freestream (assume STP conditions) with a speed of $v_{\infty} = 100$ ft/s. The total pressure would just be $p_0 = p_{\infty} + \frac{1}{2} \rho v_{\infty}^2$, but what I fail to understand is how we would get the static pressure. Would it just be the freestream pressure? Or something else...

The pitot tube is pointed into the airstream and measures the total pressure. Static pressure must be measured, not calculated. It is done so at a separate location, using a static port.

Both pitot tubes and static ports are needed in order to derive air speed, as this incident sadly illustrates.

Some pitots also have a static probe, they are known as pitot-static probes. Examples.

So there is no mathematic expression for it?

No not really, pressure is just the average impact energy of the Brownian motion of air molecules, a statistical entity rather than a mathematical expression. It needs to be measured, not derived.

@mins Yes static pressure can be measured at the pitot tube as well, as long as a location is chosen that is relatively insensitive to air stream velocity.

Yes that is always the answer I have seen too... The issue I have always had with it is, how do you find a spot that has "static air pressure" on the surface of an aircraft going 500mph. On the side flat surface of the fuselage, Venturi should dictate that air is sucked out of that hole. Similarly, any trailing edge will be more on the vacuum side. More-over, what happens when you are crabbing in on a cross-wind approach.

@Trevor Both the static ports and the pitot tube are not insensitive to how the airflow shapes over the aircraft, no matter where they are located. During certification of the aircraft, a separate pitot tube with wings is dragged behind and below the aircraft, in order to take measurements out of the flow field of the aircraft. Data from the drone tube are then compared with the pitot/static ports installed on the aircraft, and used to calibrate the installed instruments. As to sideslip, there are static tubes on both sides of the cockpit - average indication is calibrated as well.

@Koyovis ah.. interesting.


text:
Under what circumstances can Part 91 allow compensation for air transportation?
Can a commercial pilot fly passengers for hire without an air carrier certificate, considering that the aircraft is owned by a private company that hires the pilot and the plane has room for 5 passengers and 2 crew?

Related: What do the terms 'holding out' and 'common carriage' mean? and also What are the aircraft requirements to legally offer commercial sightseeing flights in a C-172?

The only way you can carry passengers for hire as a common carrier without an air carrier cert (either 121 or 135) is the sightseeing exemption -- 25 SM, landing at point of takeoff. However you will still need an LOA and to participate in an approved drug enforcement program.
If there is a 'commonality of purpose' e.g. all the passengers are employees of the company that owns the airplane, then it isn't common carriage, and you don't need the air carrier cert. This is how corporate flight depts operate, they're Part 91. (But they can only fly employees etc., they cannot fly the general public.)


text:
How to calculate Vx for a propeller driven airplane?
I know that Vx for jet engines airplane is at Vmd and that Vy is at 1.32 Vmd;
I do also know that Vy for propeller airplanes is at Vmd.
Now I cannot figure out where i can get the Vx for propellers.

Do you mean $V_{MD}$ or $V_{MC}$

Vmd (minimum control speed)

The ratio between max. range and max. endurance speeds is always 3^0.25 = 1.316...   It's the same for prop planes, jet planes, or gliders.

v$_{mD}$ is minimum drag speed, when induced and zero-lift drag are equal.


text:
How do you figure out which frequency to contact approach on for a class bravo transition?
I'm about 20 hours into my PPL, so maybe I just haven't gotten to this part yet.
Suppose I'm flying KFCM to KRNH and I want to request clearance across the bravo. How do I figure out which approach frequency to contact KMSP on?
From liveatc.net, it looks like approach is 121.2 or 126.5 (for low altitude). At skyvector, 121.2 and 126.5 are listed under Class B. From the Chart Supplements - neither of those frequencies appear. Am I just looking at the wrong information?
I found a similar case for KPBI where the approach frequency (124.6) doesn't show up on the chart supplemental.


text:
How much thrust is required for a Boeing 747 to "keep flying" at 4 km altitude?
Not full power, just enough to keep the plane moving at 4km altitude.

This borderline qualifies as a valid question, but please add some context. Are you referencing a specific event?

The QRH (Quick Reference Handbook) contains a few tables for flight with unreliable airspeed. The one for holding comes closest to the situation you asked about:

(Boeing 747-400 QRH 20.2 - Performance Inflight - CF6 Engines)
As you can see, holding at 10000 ft altitude requires between 59.7% and 77.4% N1 depending on weight. You asked about 4 km, which is about 13000 ft, so a little more will be required.
There is also a table for cruise speed at various altitudes:

(Boeing 747-400 QRH 20.1 - Performance Inflight - CF6 Engines)
You can extrapolate the values shown here between 10000 and 20000 ft altitude to get the required thrust at 4 km. Note however, that the speed flown here is higher (290 KIAS or Mach 0.84) than the one required for maintaining altitude.
If you combine the information from both tables, you should get a good idea of the required N1 in your situation. The number will however strongly depend on the weight of the aircraft.

Minimum thrust is at vbg, does the qrh have a section on that as well? Might be interesting to compare to the speeds given here.

@Sanchises No, not as far as I could find. Vref30+80 (reference speed for flaps 30° + 80 kt) should be relatively close to the safe maneuvering speed in clean configuration. It is definitely possible to fly slower, but probably not that much.

There should be min drag speed tables which represent best L/D for cruise.  They may not be in the QRH however.  If not, I'd look for the driftdown tables which are best L/D for engine out descent.

@JohnK Yes, I saw the driftdown tables, but they don't go down to 13000 ft. With just one engine inop even at the heaviest weight, the driftdown altitude is still 23000 ft (at ISA+15). The driftdown speed would be 309 KIAS in that case.

@Bianfable could you help me and say how much steam and at what pressure is required to have 2KN of thrust for 20 seconds? I post it as a new question but was closed and said that not valid question for this forum.Thanks

An rough estimation follows: in s/l flight, lift = weight and thrust = drag. If we assume a mass of 350000 kg and an L/D of 15, the thrust will be (350000 · 9,8)/15 = 228700 newton...

This 220kn (approxi) is around 15~25% of total thrust?


text:
What is the impact of lack of stereoscopic vision on a first class medical in the EU?
I have just completed aero-medical examination for first aviation class. I was refused due to the lack of stereoscopic vision. In the book with squares with circles in 3D, I saw only 4/9. The medical examiner said that I will get class 2 and if the aviation school issues a certificate that I can take off and land without any problems, he will "try" as an exception to approve class 1.
Does it make any sense to spend money and waste time, or am I already deleted? What are my chances?
PS. I'm from Poland.

In Canada you can hold a Cat 1 with vision in only one eye, so I would think it's worth a shot.

What are your goals with class 1 medical? Just recreational flying, or pro?


text:
How to identify transmit labels for a particular equipment in ARINC429?
As i can see, each equipment has a set of labels which the equipment may supposed to be received. Say if i am Equipment 03Ch (Tire pressure monitoring system), i will be configured to receive below labels
LABEL 060 -> Tire pressure (Left Outer)
LABEL 061 -> Tire pressure (Left Inner)
LABEL 062 -> Tire pressure (Right Inner)
LABEL 063 -> Tire pressure (Right Outer)
LABEL 064 -> Tire pressure (Nose)
But how may i know, what are the labels should i send ?

The Equip ID identifies the source of the data labels, not the consumer. Equip 03Ch will output your list, not receive them. The TPMS measures the tire pressures and sends the data to the onboard maintenance and cockpit display systems.

Looks like a duck, quacks like a duck, perhaps you could report that comment as an answer?

As Gerry mentioned in their comment, the equipment ID label (377) indicates to the receiver how to decode the labels being received on that ARINC 429 channel.
The transmitting equipment (Tire Pressure Monitoring System) would send label 377 with the value 03C at it's appropriate rate (usually every 1000 ms) and then labels 060 through 064 at their appropriate rate.
The receiving equipment gets the Equipment ID from label 377 it received and then knows to decode labels 060-064 according to the TPMS definitions.

So can i say, 060-064 are the labels transmitted by equipment 03C (Of course label decoding depends on equipment ID). My actual question is Let say i am Equipment 03C, what else are the labels should i transmit (or) i always act as a receiver ? Should i get a table that differentiates transmit and receive labels for particular equipment ? Thanks in advance.

ARINC 429 is unidirectional, so unless you have a separate ARINC 429 Rx channel, your 03C equipment will only be a transmitter. I didn't see any other labels for 03C in the spec, so any additional labels you transmit will be product defined (you define them)


text:
What is the yellow marking underneath Delta A220 doors?
I've recently noticed a yellow marking underneath the left door of Delta A220s. It's a yellow rectangle with a black T on it and it is exclusive to Delta and no other airlines on the A220. Any ideas what it's for exactly?
Thanks!

This is almost certainly an alignment marking for the passenger boarding bridge (jetway). You can find the same markings on other Delta aircraft as well:

(image sources: A320, B737, B767)
The dead giveaway is that these markers are found on the second door on the long haul jets, which are typically boarded through this door instead of the first one:

(image sources: A330, B777)
The jetways used by Delta will have a similar marking on them which aids in properly aligning them with the aircraft door such that the door can still be opened and passengers can safely board and disembark.
See also: What are these markings under the passenger door?

I'm just guessing, but it could be a guide for positioning the  steps, especially as ground crews are still getting familiar with A220.


text:
Why does JetBlue have aircraft registered in Germany?
I recently saw a JetBlue A321 with a German flag on its tail and was wondering why it would be registered in Germany if it was a US-based carrier. I was thinking maybe it had something to do with Airbus but I wasn't 100% sure.
Any thoughts?
Thanks!
Image source: https://www.linkedin.com/feed/update/urn:li:activity:6771365770770309120/

Possibly related

JetBlue was at one point partially owned by Lufthansa.

D-AUAA to D-AZZZ are test and delivery registrations. The registration is now N2105J.

I am the pilot who delivered this aircraft on Friday.
The German registration remains on the aircraft up until the time we (JetBlue) purchase the aircraft. Prior to transfer of title, we perform a series of ground tests and flight checks.  This is all accomplished while Airbus still owns the aircraft.  Once any discrepancies are corrected, and we accept the aircraft, we buy it, the ownership changes and the temporary (German) registration is removed. This is typically the day prior to ferrying it over to the US.
As for the Hamburg/Toulouse question- TLS does not produce A321 aircraft, so all of our JetBlue deliveries come from either Hamburg or Mobile, AL.

Now that is what we can say is an authoritative answer! Thanks for resolving this question, and welcome to Av.SE!  Please come back often; we get quite a few "why does ___ work this way on an A32X" questions.

Is the A321's range long enough or did you have to refuel en-route?

@DavidBrossard that would be a good new question to ask

@DavidBrossard It looks like it was direct; 8:10 long: https://i.imgur.com/aF0Ul9r.png

@DavidBrossard The A321 even lists 4000nm as operational range which puts the ~3800 nm from Hamburg to New York within the possible range even for normal flights.

Fun fact: they can't build the A321 in TLS because it won't fit through the door, at least until they convert the A380 line over.

That is a very very new aircraft. The notes on the jetphotos.com posting (photo taken 2/21/2021, uploaded 2/27/2021) say:

the first A321neo for jetBlue with the Mint suites on board for North American flying and the new "Ribbons" tail design...delivered 26.02.2021 as N2501J

Delivered only yesterday!
The N-number must be a typo, because N2501J is registered as a Cessna 150 and that registration doesn't expire until August 2022. Happily the posting tells us the manufacturer's serial number, 10101, and the FAA's N-number lookup tool allows a search-by-serial among other options. We see that JetBlue is the proud new owner of MSN 10101 N2105J, recently arrived in New York. I don't know when the stickers removal takes place but I suspect it happened before the aircraft left Germany.

Zoom in and contrast/brightness adjusted to reveal N2105J underneath the sticker

The general idea is that the German registration is only temporary until the aircraft is re-registered under its new owner. In fact looking at all jetphotos.com entries for D-AZAJ we can see that specific registration seems to be a relatively common one for fresh-off-the-line A321s: June 2009 in US Airways livery, February 2013 Turkish Airlines, August 2013 Air China, April 2019 Air Transat, December 2019 Starlux, February 2021 JetBlue—all different serial numbers.

Also note the photo location:  Hamburg Finkenwerder - EDHI
Germany. That is inside the Airbus Hamburg factory.

… I would add that Airbus paints the aircraft at Hamburg, so the aircraft received the final paint job there, but actually hands them to customers elsewhere, usually in Toulouse. Since after that the aircraft never returns to Finkenwerder, it was clearly photographed before delivery.

@JanHudec, that's interesting because the FlightAware track shows a direct flight (under the N-number) from XFW/EDHI to JFK.

Hm, I don't know. It used to be Toulouse, but maybe they reduced travel due to coronavirus and are now handing over in Hamburg, possibly with electronic signing of documents so the higher management does not need to be present.

fantastic Sherlocking here !

@JanHudec I seem to remember hearing that papers were signed in international waters to avoid taxes, although that might just be a made up story? If not, has that stopped in COVID times?

@Tim, I don't think it would make any difference to the taxes according to the European law. EU does not have a sales tax, and the value added tax depends on whom you sold something and what it was but not where.

@JanHudec Where something is sold does matter for the VAT. The VAT is not an EU tax, but a national tax, and the rates in France are probably different from Germany. But it doesn't matter here. VAT is only charged for domestic sales; it is refunded when you export the goods (even as a non-EU tourist, you can get that refund, although there is some red tape involved). Since JetBlue is in the US, the VAT would probably not even have been charged in the first place.

Also interesting in this context is https://aviation.stackexchange.com/questions/69726/how-come-some-aircraft-have-really-cool-registration-numbers?rq=1 - apparently, D-AUAA to D-AZZZ are set aside specifically for Airbus Finkenwerder.

@JanHudec I think it's because they built a fancy delivery center for the A380 in Hamburg, and with that program ending, they might as well use the infrastructure.


text:
Does anyone have photos of internal parts of ELAC, FAC, SEC or Boeing 777 flight computers?
I'm an electrical engineer and I'm interested in civil aviation, more specifically in fly-by-wire planes like the Airbus A320, Boeing 777 and others. But I fell in love with the Airbus A320 as it is the first civil fly-by-wire plane. I read a lot of its flight computers (FAC, SEC, ELAC), I saw its photos. But I want to have a look at their internal parts.
Does someone have photos of these internal parts? I want to have a look at those computer's internal boards. At last, I would appreciate any deep technical overview of how they work.

This is an indirect answer at best, but I think you'll get something good out of it.
I would definitely recommend you have look at CuriousMarc's series on YouTube where he and a couple other SMEs restore an original Apollo Guidance Computer to working order. You might think "that's not civil aviation, why?", but the AGC was the first digital fly-by-wire system. It was revolutionary both in terms of it's use of silicon integrated circuits and it's effects on software engineering. the AGC was also the basis for the first fly-by-wire systems installed on aircraft for atmospheric flight.
The other reason I'd recommend this is because it has all of the electrical cutting and shutting that you seem to be expecting. They spend a lot of time looking over original schematics and performing diagnostics with Marc's extensive personal collection of equipment in order to find individual failed circuits which require repair. They also had to find ways to interface the computer and emulate missing components, which involved a lot of bespoke design.
I'll be honest, I think there is a possibility you might find modern computers quite boring in comparison if your interest is limited to the circuits and does not include the software (not easy to tell from the wording of the question). This is for two reasons:

The AGC represents a time where the atomic components of the digital computer could still be seen with the eyes and cut/soldered out by hand. That is almost certainly not the case with modern computers, and I would also expect that the vast majority of people who work with the aircraft directly have never cracked open one of the computers for this reason. They are sealed (maybe even tamper-proofed), and are simply replaced as a unit and shipped back to the OEM when any invasive work is required.
A cursory search informed me that the original A320 computers actually used derivatives of the Intel 8086. If you're interested in early x86 micro-architecture (especially in fault-tolerant configurations), this is interesting. But, if you're expecting to open the computer and find the flight control laws to be implemented in hardware, you'll be a bit disappointed.

The big sea change in FBW has been the move of the analog-to-digital conversion of signals from inside the controller to out near or on the actuator.  One of the big downfalls of analog FBW was the need to send sensitive analog low-voltage signals down long wire runs with numerous connectors, with the need to account for voltage drops in calibrations of signals.  Anything that affects the line resistance, like bad connector pins, plays havoc with the system.  Doing the digital conversion on-wing means the connection just has to be good enough for the signal to get there as an on-off.

Thank you very much for your answers!

user44968, you gave me very interesting paper to read! What a pity that those who work with aircraft have never opened any computer. But the reason is very well understandable.

A little late but maybe I can help you.
The Spoiler Elevator Computer is based on, in total, four CPUs. Two 80186 and two 8086. Airbus computer in general, or at least in case of FBW computer, are based on a redundant architecture. They consist of a command and monitoring channel. As the name implies, the COM channel processes flight control laws and does servo control. The MON channel processes flight control laws, but there is no servo control, instead it monitors the position of the surfaces, namely spoilers plus in case of ELAC fail the THS and elevators.
Now some details:
COM and MON are physically separated in the computer housing. In the center of the LRU are four slots where a PSU modul can be fittet. Two of them are used. One PSU modul for COM and one for MON. The slots separate the COM and MON Boards. COM consists of three boards: Main CPU 80186, Servo CPU 8086 and a Servo/IO board. MON the same but without Servo/IO. The CPU Boards are identical, there is no difference between COM and MON. Software for the  Main CPU is stored in an OBRM (128 KByte), Airbus terminology for a ROM Module. When you saw outside pictures of the SEC: the OBRMs are the labeled cartridges. They are directly connected to the Main CPU board. Software for the 8086 is stored on two socketed ROMs (64KByte together) on the board. Servo and Main communicate via a dual port ram. Servo jacks or rather servo valves, related LVDTs and so on are connected via relays located on Servo CPU and Servo I/O. For THS and elevator, COM and MON relays are in series. For spoilers relays are located on Servo IO (if I remember correctly) and controlled by MON. Relays can be controlled by the CPUs or the Engage Logic. The engage logic is a set of PLDs with basic logics implemented, taking over even when all CPUs aren't working. For example, the Main CPU is monitored by a watchdog timer. It has to be periodically resetted, or it will trigger the engage logic, which in turn will switch the relays. I think even the SEC push button on the overhead panel is connected to the engage logic and the small light which will illuminate in case of failure.
Arinc429 output is partly done by the CPUs, they generate four bytes with data and label included by software, and then push it into a PLD byte after byte, which then does parity and parallel to serial conversion plus arinc adaption (RZ Coding).
One transmission PLD is fitted on Main and one on Servo. Main sends data to the FCDC and Servo should process the output to the opossite channel (Arinc429 Cross Talk Bus).
Arinc input is managed by one set of PLDs on the Main CPU board. They validate the data and load it into a dual port ram. Not the same dual port ram as mentioned above for main-servo communication! The "hardware driver" software on Main loads byte after byte from the dual port ram into its own ram and does final depacking. (mostly just a few bit shifts in case of air data and so on and shifts plus masking in case of discrete words where each bit represents a state)
For discrete IO adaption circuits are used (changing voltage levels) and then inputs are multiplexed on the 16th bit of the x86 data bus. Output the same just the other way around.
All servo related hardware is located on the Servo CPU and Servo IO board. LVDT and RVDT signals are demodulated in hardware and then read in by the 8086 via a "diy" integrating ADC.
Analog input from potentiometers like sidestick, thrust lever and speedbrake lever are processed by the 80186. They are multiplexed onto a single ADC.
On the Servo IO board op amps for driving and regulating the current commanded by the 8086 through the servo valves are used. A set of four dual DACs drive the op amps.
Momentary current of each servo output can be read back, also by the 8086, to verify correct operation of the analog servo current control.
Maybe I forgot something but for the beginning, that should by much information. Basically the SEC is "just" a big discrete build MCU, or let's say two MCUs (COM and MON).
Hope I could help...
Simon

I always thought it was 80186 and M86000 (so it is dissimilar).

The Motorola 68k is used in the ELAC. So there is a dissimilarity between ELAC and SEC.

That massive paragraph in the middle could use some formatting to make it readable.


text:
What are the contributors of lift-dependent drag?
If someone wants to plot the drag polar of an aircraft, we have
$$ C_D = C_{D,o} + C_{D,i} $$
form/skin drag plus the induced.
However, as I feel, form drag and probably skin friction drag (parasite drag) also lift-coefficient dependent. For example,
as the angle of attack increases, the shape will be different and so the parasite drag will be different  .
Does that increment in parasite drag considered in the induced drag term because it's lift-dependent?
Where should we add this contribution of drag in terms of
the drag polar formula? in the lift-independent part or in the lift-dependent part?

It is even more complicated than that. When the wing stalls, the lift decreases, but the drag, and this is clearly form drag, increases significantly.

If someone wants to plot the drag polar of an aircraft, we have
$CD=CD_o+CD_i$
form/skin drag plus the induced.
Something to note here is that there are more than one mathematical "model" to describe the relationship between lift and drag. What you have described is known as "Parabolic Drag Model", where we consider a parabolic relationship between lift and drag.
Does that increment in parasite drag considered in the induced drag term because it's lift-dependent?
Imagine an airfoil drag polar (2D polar so there is no induced drag taken into account). If this polar can be closely modelled as a skewed parabola (skewed due to camber) then $e$ in the induced drag term $\frac{CL^2}{\pi*e*AR}$ takes care of this drag rise up to some extent. This kind of a 2D airfoil drag model is usually applicable for mostly turbulent airfoils such as NACA 4 series in high Reynolds number flows.
Where should we add this contribution of drag in terms of the drag polar formula? in the lift-independent part or in the lift-dependent part?
Above said, mostly laminar airfoils such as FX163 or NLF series airfoils which operate in relatively low Reynolds numbers does not fit the above model fairly well. These airfoils exhibit a linear relationship between lift and drag at-least for the operating range which is not affected by massive separation. So in this kind a scenario its best to modify the above equation with a linear term as well.
$CD=CD_o + k CL + \frac{CL^2}{\pi e AR}$


text:
How close is a pilot allowed to get to a runway without landing clearance?
I heard it was 100 feet above minimums, I'd just like verification.

By the way, "above minimums" doesn't mean anything to a VFR pilot.

@RonBeyer Granted, I'm not a pilot.

Don't VFR guys have minimums? @RonBeyer

@Harper-ReinstateMonica Sure, but not the way you'd think about "minimums" in this context. For example at a Class-C airport you need to be 500 feet below, 1000 feet above, and 2000 feet horizontal. I've been well below 500 feet getting clearance to land on many occasions, 1000 feet AGL is pattern altitude... When working in the area of an airport, especially coming in for landing, "minimums" is an IFR term.

From the Pilot/Controller Glossary:

CLEARED  TO  LAND−ATC  authorization  for  an aircraft to land. It is predicated on known traffic and known physical airport conditions

A clearance to land is exactly that: a clearance to land. A pilot without landing clearance hasn't violated ATC instructions until their aircraft has, in fact, landed. So the answer to how close you can get is: anything that isn't touching the surface.

That said, if you're not cleared to land and you skim the runway of a controlled airfield at 100ft AGL there will be phone calls.

@J... Depends whether one executes a proper go-around or rather a low approach. The low approach requires a clearance the same way landing or touch and go do.

But what about the minimum distance? No word about that? But still the right answer?

Disregarding all other factors (like weather etc) an approach can be made as low as you can without touching the pavement.
It is not uncommon in major airports with a lot of traffic to get landing clearance very low, well below approach MDA or DA.

The question is not about a low approach

But I'm sure you had clearance "for the option" before doing that... The op isn't asking about go-arounds...

Edited the answer

TL;DR there is no minimum distance
An aircraft landing on a runway of a controlled aerodrome may do so for a variety of reasons.
For example they might be on final approach and due to communication loss they don't get a final clearance and land anyway.
In a controlled aerodrome you are usually given a clearance to enter the TMA (terminal movement area) as well as a STAR (standard arrival route). Entering a TMA without clearance will surely cause more trouble, especially at busy and / or sensitive aerodromes.
There are even incidents, where pilots landed on the wrong runway or even the taxiway. Also landing at the wrong airport in the same TMA has happened.
Landing on uncontrolled aerodromes is also regulated. There is usually a frequency to listen to and a pattern to fly, to make sure you are the only one landing.

This doesn't answer the question... Quite often you are given clearance to enter the pattern or fly towards a particular runway, but the arrival frequency may not pass you off to tower or tower may not give clearance until short final. The OP is asking how close you can get to the runway without tower's clearance to land.

It does answer the question in the first sentemce.

The question is about the minimum distance from the runway, there is no such thing. So how does this not answer the question?

authors: Vladimir F Героям слава


text:
How do single engine airplanes handle changes in torque?
In multirotors, there is a regulation of torque from the rotors which keeps it stable. However, in single engine airplanes, there is only one engine with one propeller.
How is the aircraft able to stabilize itself in such a case?

Are you asking about planes (i.e. single-engine fixed-wing aircraft) or about a single-rotor drone (you tagged [unmanned-aerial-vehicle]), like helicopter?

Yes I am asking about plane only. Specifically single Engine placed at nose of the plane.

OK, I edited your question accordingly. Feel free to edit again, if something is missing...

Answer: Depends on the condition of operation.  For ground ops or the takeoff roll, torque effects are usually countered using rudder inputs.  During flight, light airplanes are sometimes designed with increased washout in one of the wings to counteract engine torque using lift asymmetry which is applicable with their limited flight envelopes.  More powerful single engine airplanes have dedicated aileron trim, which provides a more effective counter torque solution for the full flight regime.

it seems to me that the rudder is not centered and therefore cancels this effect.

Single engine airplanes experience torque proportional to the amount of engine power used. Pilots mitigate this torque by applying engine power smoothly and gradually. Since the torque on most planes will be applied to the airframe in the counter-clockwise direction from the point of view of the pilot, the pilot further mitigates this torque by applying Some right rudder when applying engine power. Aircraft manufacturers mitigate torque by offsetting engine placement and control surface neutral positions slightly. Quick applications or reductions in engine power will still cause the torque to be noticeable enough to create a left turning tendency.

Did you mean to say applying right rudder when adding power?

Yes, I did. Thanks for the catch.

Flight instructor's mantra: More right rudder! More right rudder!

“Flying is expensive. Right rudder’s free” - local DPE.

The rotation of the powertrain (or GMP) consisting of the engine and the propeller causes what are called engine effects. These effects depend on the power supplied by the engine, the speed of rotation of the propeller and the speed of the airplane, for a given airplane.for right-hand propeller, that is to say which rotates clockwise when viewed from the pilot seat.
The overturning torque or engine torque is an application of the principle of Newton's third law "action / reaction". Any action produces an equal reaction and of opposite direction ". By reaction to the movement of rotation of the propeller (action), an antagonistic couple tends to cause a rotation of the whole of the plane in opposite direction (reaction). overturning moment is more important phases of flight when the engine is operated at full capacity, particularly during takeoff or overshoot phase.
If this couple is very important on single-engine airplanes with high motor power (Siptfire, Messerschmitt Bf 109 etc.), it is not very sensitive on modern weakly powered aircraft.

This overturning torque is generally compensated during the design of the device for a given engine speed and a given speed (speed and cruising speed in general):
- either by increasing the pitch angle (incidence) d 'half a wing,
- or by increasing the surface area of a half wing.
That is to say by increasing the lift on a half-wing. For the other flight phases a fin compensator (tab) or the stick will be used.
any increase in lift leads to an increase in drag. So the left wing will have a greater drag than the right wing (right-hand propeller). This difference in drag will generate a yaw moment on the left. This yaw moment can be corrected by:
- a slight timing to the left of the vertical fixed plane.(or compensator)

Edit:there is other torque like the roll moment,the yaw moment....
Source: https://www.lavionnaire.fr/HeliceEffetsMot.php

Most single-engine types are stable and low-powered enough that torque is not a problem. The pilot will compensate in the usual way if one wing drops. A pilot may also trim the ailerons to counteract it during cruise, so that the plane flies level without any extra control input.
Historically, it was manageable enough on the early pioneer aircraft until the Gnome rotary came along in 1909. This engine not only had much greater power than anything else its size had had previously, but the whole engine spun round adding gyroscopic effects on top. Unstable planes with bad habits were the norm in those days, but still had to learn to be extra careful.
When J. W. Dunne fitted a second-generation 70 hp Gnome to his stable D.7 tailless swept monoplane in 1911, the torque was enough to destabilise it. He fitted a "torque flap" to the wing on one side and would raise it during landings, which had to be "dead stick" with the engine off because it had no throttle control. His larger D.8 biplane was stable enough to accommodate the Gnome quite happily.
During WWI the Sopwith Camel scout fighter had an even more powerful rotary engine, and became famous for its ability to turn like lighting in the one direction but only sluggishly in the other. The problem became insignificant when the rotary engine was abandoned.
With the advent of ever-higher engine powers in WWII the problem came back. Some designers added bigger tails and/or aileron trim tabs, a few made one wing longer than the other, pilots grew cautious of over-revving the engine on the ground.
Perhaps the ultimate solution, which saw some use well into the postwar era, and on turboprops as well as piston engines, was the contra-rotating propeller. Typically a three-bladed prop spun in one direction and another, immediately behind it and on a concentric shaft, spun the opposite way. Thus torque was greatly reduced. Both were geared from the same engine, with one exception: the Fairey P.24 comprised two 12-cylinder engines sharing a common crank case and each driving one of the two props, an arrangement in which half the engine could be shut down in flight to greatly improve endurance. However the P.24 was dropped in favour of the less-powerful Rolls-Royce Merlin and the Naval plane it was meant for, the Fairey Battle, never lived up to its potential.

As far as i know, the plane is not stabled, pilots always have to stablilize the plane by themselves when they push the throttle up.
In fact the effect of the torque of the engine is not big enough to significantly effect the plane's flight, although it is noticeable on some planes.

I find the torque most noticeable immediately upon the mains leaving the ground after rotation at full throttle. After that, spiraling-slipstream and P-factor probably account for more of the left turning tendency. Although, in the air, I do feel the torque more with a C182 than I do with a C172. Especially if I advance the throttle too quickly. Say, for instance, in a go-around.

you should correct the English (e.g. the verb for stable is stabilize).


text:
How are the BNR and BCD data formats identified in an ARINC-429 message?
I'm a newbie in ARINC-429 standard. How does the receiver identify which data is in BCD format and which is BNR? I understand that labels indicate the message data carried information, but it does not indicate if the data is BCD format or BNR. If my understanding is correct, the BCD or BNR format used is dependent on the equipment ID. And that leads to my next question: how is the equipment ID embedded in the message?

Welcome! It think it doesn't know from the data, the format is published either in the standard, or by the LRU manufacturer. Related: What is the encoding difference between ARINC 429 BNR and BCD formats? and also this LRU document page 18 for an example.

The receiver has to be programmed.  It's important to remember that A429 is a unidirectional hard-wired bus with a single transmitter and multiple receivers.  Each connection is physically unique and the reason for all the pin-out diagrams and bus definitions in the ARINC Characteristics.
To help with data integrity and fault detection, each transmitter includes Label 377 on the bus which contains its equipment ID.  For each type of equipment, the label assignments and coding are fixed.
So, if you're building an LRU that's receiving GPS data, you would program the port assigned as the GPS#1 Input to verify the equipment code is 00B hex and the find the specific labels your LRU needs (it's likely to be a small subset of all data on the bus).  Since you know the source LRU and the Labels you're looking for, you know the individual label coding and can program the receiver to decode the data.
Most avionics companies have developed custom ASICs to act as the A429 UART.  It simplifies the design and saves costs.

The answer is the first statement, but it could be expanded to say: The receiver doesn't know, it has to be programmed.  The developer has to look up the standard and program each port to find and decode the labels it needs.

Thanks, I saw that 00B hex equipment ID for GPS uses the BNR format for various labels ID according to their respective operations in page 15 A429 specs, Table 1. No BCD format for GPS. Cool!

Assume that you are reading data from a LRU
that LRU is capable is sending different labels
You should program such a way that which label is corresponding to what type of decoding should be done.
In general this information will be present in the interface communication document.
Like label 05 is encoded in BNR
Label 06 is encoded in BCD
It is predefined.
Hope you understand


text:
Are Boeing 737 winglets adjustable from the cockpit?
As I taxied past some Southwest 737s during a recent trip I saw what appeared to be different angles of the winglets relative to the wing. Are winglets adjustable, are they arranged differently from Port to Starboard, or is it an optical illusion?

Winglets are fixed to the ends of the aircraft. As can be seen below, the winglet has no movable parts and is fixed to the end of the wings.
http://www.sae.org/dlymagazineimages/2861_2313_ACT.jpg
Boeing 737 winglet; image from sae.org
What you'd seen is an optical illusion. The port and starboard side winglets would not be much different, usually (except that they are mirror images externally).
Interestingly though, Boeing has filed a patent for an adjustable winglet. Airbus is also working on a similar project, called the morphlet.Both of these are geared towards reducing drag and saving fuel.

What are the wires for? Do they transmit data from some sensors?

@DmitriPisarenko Winglets usually have nav lights; the wires are for that.

You may have just seen aircraft with different models of winglets. According to this site there are four different models of winglets.

737-200 Mini-Winglets
737 Classic/NG Blended Winglets
737 NG Split Scimitar Winglets
737 MAX Advanced Technology Winglets

All winglets are permanently fixed to the wings and are not adjustable.

737 winglets are not adjustable. There are several configurations in the fleet, as noted by @TomMcW, but they are all fixed. Airbus and other commercial aircraft winglets are also fixed. The only winglets with control actuation I've seen in actual use are on "flying wing" configurations like the Swift:

Studies (e.g. AIAA 2003-4069) show that for classic (tube-and-wing) aircraft, incorporating control deflections into an otherwise optimized winglet reduces the best lift to drag ratio.

737 winglets are not, to my knowledge, adjustable. However, some aircraft do have adjustable winglets, like the XB-70

No.  The winglet is a fixed structure.


text:
Is there a database of airfoils for a given Reynolds number?
I'm trying  to design my ultralight airplane. I know the basics of aerodynamics and airplane design, however, my question is
if there are any tools (online, software, database ) that suggest the best set of airfoils with best possible Cl/Cd ratio based on a given Reynolds number?
I think that the database  should have all airfoil graph data and not just the geometry information.

Did you try to search the net? Please let us know what is missing from what can easily be googled.

@PeterKämpf  as I mentioned I already did my primary design but I've never found any tools that have the capability to search inside all airfoil aerodynamics data. The online tools that already know, they just give the aerodynamics data for an airfoil that selected

@Hamid s k May I suggest the choice of airfoils for a practical ultralight are limited and known, so you are best served by studying existing designs. Dimensions, weight and speed, there for Reynolds, will be in a fairly narrow range.

The parameters of interest are too varied to make this a topic for a database search.  I do not know of any airfoil database which would allow what you want. The most you can expect is to search for a given thickness and camber. Which thickness or camber would be best its up to you to decide – a database cannot do that.

@RobertDiGiovanni, PeterKämpf, thanks for the advice, actually, I'm trying to make software design with python that its input is dynamic & geometry constraint and I look for airfoil aerodynamic repository data to estimate the best possible selection, but most the tools I checked, made the airfoil selection manually

resource location is of topic as describe by the help cente.

Reynolds Number is calculated as:  velocity × chord/kinematic viscosity of air
velocity is meters/second, chord is meters, kinematic viscosity of air is around $1.48 × 10^-5$
Then go to Airfoil Tools.
As you make your selections, you can see, in the polar diagrams, the effect Reynolds number has on the lift and drag properties of your wing.
A typical ultra light will have a Reynolds Number around 500,000 to 2 million.  A possible path to a solution may be to study aircraft of similar size and speed range.  These have been in existence for more than 100 years.
You may find a heavily cambered rectangular wing (Aspect ratio around 6-8) with a blunt rounded leading edge will give good low speed performance and very docile handling characteristics.  Variations at this Reynolds range will be mainly in amount of undercamber you choose,  depending on speed range you prefer and engine power.
The NACA 4418 may be a good place to start.

thanks for the answer, but my question was something else, I already calculate my
Reynolds and look for a tools that look into the airfoils aerodynamics data graph and select a possible set for a given Reynolds, not show me aerodynamics data for what airfoil that I select

@dalearn this response provides an answer in paragraph 4.  Your experience as a "computer programmer interested in aviation" may benefit from following this advice.

Airfoil choice is more than "best L/D" at a specific but unknown Reynolds number. Typical questions are:

At which lift coefficient is the best L/D achieved?
What is the maximum lift coefficient at the low speed Reynolds number? A higher lift coefficient allows for a smaller wing which produces less friction drag at high speed. The isolated value of best L/D might lead you to a higher drag overall.
How does the airfoil behave in a stall? Abrupt lift loss would be a heavy price for a slightly better L/D, so a compromise must be reached.
What is the minimum drag coefficient at low lift coefficients?
What material and process is used for building the wing? Will it be smooth enough to reap the benefits of a long laminar boundary layer so the quoted L/D can actually be reached?
What is the airfoil thickness? This determines structural mass and stiffness of the wing spar. Having a lighter wing might be worth a slightly worse L/D since less lift must be produced.